The present invention relates to a method for determining the density of a bulk material at a plurality of points within a stockpile, and more particularly, to such a method that utilizes a nuclear depth density gauge for determining the density of the material at the various points throughout the pile.
Electrical generating companies and other businesses frequently keep in inventory large quantities of coal, stored in outdoor stockpiles. For business planning, financial reporting, regulatory and other reasons, it is often necessary to determine relatively accurately the quantity of coal contained within a stockpile. The quantity is customarily expressed in terms of its weight, but it is clearly impractical to determine the quantity by physically weighing the coal. Thus, the typical method relies instead upon determining the density of the coal and the volume occupied by the stockpile, from which the weight of the coal in inventory may be calculated.
Several methods are known for determining the density of coal within a stockpile. Since the density is not necessarily uniform throughout the pile, any method must rely upon a plurality of density measurements taken at various locations throughout the pile. In one method, known as the volumetric displacement method, a cylindrical rubber bag filled with water is used to determine the content of holes augered in a number of locations throughout the pile. The coal cuttings from each of the holes are collected and weighed. The volume of the hole from which the coal cuttings are removed is determined by placing the bag within the hole and filling the bag with water. The volume of water to fill the hole is recorded, and from the volume and weight of the cuttings, the density of the coal at that location may be calculated.
The displacement method presents a number of disadvantages. Measurements can be made only relatively near the top surface of the pile, and no density variation with increasing depth can be detected. The coal cuttings must be very carefully collected during augering, since any loss would affect the accuracy of measurement. The method is generally time-consuming and awkward to perform since, for example, the water placed into the rubber bag must be pumped back out prior to removal of the bag for subsequent measurements.
A second method for measuring density utilizes a nuclear depth density gauge for measuring density at a number of points in the pile. The gauge includes a probe consisting primarily of a source of radiation and a sensing element or detector. The detector is connected by a cable to a recording instrument or scaler. The density measurement is performed by lowering the probe through access tubing placed within a hole to the desired depth. The probe effectively measures the density of a generally spherically shaped volume approximately 5 inches in radius.
The detector within the gauge probe receives gamma radiation, the amount of which is recorded by the scaler. The probe source is a radioactive material that emits such radiation at a constant average rate. The gamma rays interact in various processes at the atomic level with the surrounding medium. The number of interactions, or scattering events, per unit time is a function of the density of the medium. The determination of sufficient quantities of the back scattered radiation within a certain fixed energy range and on a per unit time basis will give a statistically significant measure of the relative degree of scatter by materials of different densities.
One problem in using the nuclear depth density gauge is providing for insertion of the probe into the stockpile. A hole must be formed, and an access tubing inserted therein, into which the probe is placed. The probe is designed for use with 1.9 inch inner diameter, 2.0 inch outer diameter aluminum tubing, and the gauge is designed primarily for measurements in soil. In such a case, augering of a hole and insertion of the tubing is a relatively simple matter. In coal stockpiles, however, necessary measurement depths can be as great as 100 feet or more. The access tubing must fit snugly within the hole to achieve accurate results, but at such depths the aluminum access tubing does not possess sufficient strength to withstand its insertion the full length of the hole.
In one known method, a hollow-stem auger is used to advance the hole to a point above where the gauge reading is to be taken. The auger is disconnected from the drill rig but is left in place within the coal. A length of steel casing, having a split-spoon sampler attached to its lower end, is inserted into the hollow portion of the auger. The sampler and casing are then driven approximately 1 foot into the coal immediately beneath the auger, and the coal contained within the sampler is removed. The sampler is then replaced by a length of aluminum tubing at the end of the steel casing, which is then reinserted into the hole. The aluminum section is forced into the portion of the hole formed by the sampler, and a density measurement is taken therein. The casing is then removed, the auger is reattached to the drill rig, and the hole is advanced to just above the location of the next measurement.
This method represents a relatively complex and time-consuming procedure, particularly since a number of measurements must be taken at various depths along each of a number of holes in the stockpile. Since the accuracy of the density determination improves as the number of measurements is increased, it can easily be seen that lack of a simple method for installing access tubing along the full length of the hole represents a significant disadvantage.
A second problem associated with nuclear depth density gauge measurements in coal stockpiles results from the fact that such instruments are designed with the expectation that they will be used primarily in soil. In preparing the instrument for any use, a calibration must be determined to convert the back scattering radiation count received by the scaler into a corresponding density value. Such calibration is performed by the instrument manufacturer, but is performed such that the instrument is properly calibrated for use in typical soils. While such a calibration is sufficiently accurate for using the gauge at construction sites and the like, coal is sufficiently different material that the factory calibration values are not usable.
One method for recalibrating the gauge relies upon the difference in chemical make-up between coal and soil. The mathematical formula used in producing the calibration curve includes a constant factor which is related to the chemical composition of the material to be tested. Thus, using coal of a known density, it is possible to calculate a new constant which may then be inserted into the formula.
One disadvantage to this method, however, is that the chemical composition and relative proportions of coal from one stockpile to another is not constant, but rather various widely depending upon where the coal was mined. Thus, for the calibration to be accurate, it must be repeated prior to density measurement in each stockpile to be tested. Since the determination of the constant must be performed under carefully controlled conditions such that all other formula factors remain constant, this becomes a time consuming, tedious procedure.
The foregoing is equally applicable to density measurements of any other bulk material stored within a stockpile.
Accordingly, what is needed is a new method for determining the density of bulk material, particularly coal, stored within a stockpile. Such a method should utilize the nuclear depth density gauge due to its ease and simplicity of operation, but should reduce to a minimum the awkward drilling technique using the hollow-stem auger. Calibration should be relatively easy to perform, and capable of performance prior to measurement in each stockpile.